A repulsion motor is a type of single-phase electric motor that runs on input AC (alternating current). It is known for its effective starting torque and reliable performance during operation. Structurally, it features a stator winding, a commutator, and a rotor resembling a DC armature. Short-circuited brushes maintain constant contact with the commutator.
Classification of Repulsion Motor:
1. Repulsion Motor:
- Comprising a single stator winding, a rotor resembling a DC armature, a commutator, and short-circuited brushes in contact with the commutator.
- This motor operates solely on the repulsion principle and does not require a short-circuiting mechanism.
2. Compensated Repulsion Motor:
- Similar to the standard repulsion motor but includes an additional stator winding (compensating winding) and a second set of brushes positioned midway between the usual brush set, connected in series with the compensating winding.
3. Repulsion-start Induction-run Motor:
- Starts as a repulsion motor but transitions to run as an induction motor at a nearly constant speed.
- It features a centrifugal mechanism that short-circuits the commutator once 75% of full speed is reached.
4. Repulsion Induction Motor:
- Utilizes both repulsion and induction principles, incorporating a stator winding and dual rotor windings (a squirrel cage and a DC winding connected to a commutator).
- This motor also includes a short-circuited brush set.
Repulsion Motor Working and Principle:
Consider 2-pole salient pole motor with the magnetic axis vertical as shown below. The armature winding is similar to DC Motor with commutator and brush which is short circuited by low resistance wire called jumper.
- When alternating current flows through the stator field winding, it generates an N-pole at the top and an S-pole at the bottom. This alternating flux in the stator induces an electromotive force (EMF) in the armature conductors through transformer action.
- The direction of the induced current in the armature depends significantly on the positioning of the short-circuited brushes. These brushes influence the interaction between the magnetic field and the current flow, affecting how torque is developed and ensuring that the motor starts and runs effectively. Proper placement of these brushes is important for optimizing performance and achieving torque.
- If the brush axis aligns with the magnetic axis of the main poles, it affects how torque is generated. The induced currents in the armature, illustrated by dots and arrows, create an electromagnetic field with a N-pole at the top (under the main N-pole) and a S-pole at the bottom (over the main S-pole). This alignment results in face-to-face positioning of the main and induced magnetic poles.
- Due to this arrangement, no torque is developed because the repulsion forces at the top and bottom act along the Y Y′ axis and oppose each other directly. This direct opposition cancels out the torque, preventing motor rotation. Proper adjustment of the brush axis is essential to create a displacement that enables effective torque development.
- If shifting the brushes by 90º to a position where the brush axis is at right angles to the magnetic axis of the main poles changes the behavior of the motor. Even though the induced voltages in the armature conductors remain the same as in the initial position, the new brush alignment causes these voltages to neutralize each other across the paths between the brush terminals.
- This cancellation results in no net voltage across the brushes, preventing the flow of armature current. Without an armature current, torque cannot be developed, meaning the motor will not produce rotational motion. This illustrates the importance of correct brush positioning for ensuring the motor's torque generation and overall operation.
How Repulsion Motor Generate Torque?
- When the brush axis in a repulsion motor is positioned so that it is neither aligned with nor at 90º to the magnetic axis (Y Y′) of the main poles, a net voltage is induced between the brush terminals.
- This voltage generates armature current, allowing the armature to act as an electromagnet with its own N- and S-poles.
- The rotor poles do not face the corresponding main poles directly. Instead, the rotor N-pole is repelled by the main N-pole, and similarly, the rotor S-pole is repelled by the main S-pole. This repulsion generates torque, enabling the motor to operate.
- The brush positioning is important for balancing the interaction between the rotor and the stator magnetic fields to generate torque.
- If the brushes are shifted anti-clockwise from Y Y ′, rotation will also be anti-clockwise. It means, the direction of rotation of the motor is determined by the position of brushes with respect to the main magnetic axis.
Important Points to be Noted for Repulsion Motor:
- Starting torque developed by repulsion motor depends on the amount of brush-shift whereas the direction of rotation will depend on the direction of shift.
- Maximum starting torque is developed at some position where brush axis makes, an angle lying between 0º and 45º with the magnetic axis of main poles.
- Speed control can be done by brush shift.
- It has high starting torque (about 350 per cent) and moderate starting current (about 3 to 4 times full-load value).
- It has low power factor at low speed and good power factor at high speed.
- It can have issue related to brush sparking.
- The speed varies significantly with changing load conditions. Operating at no load, the motor speed can become dangerously high.
FAQs for Repulsion Motor:
What is a repulsion motor?
- A repulsion motor is a type of single-phase electric motor that runs on AC (alternating current) and is known for its effective starting torque. It has a stator winding, a commutator, and a rotor similar to a DC armature, with short-circuited brushes maintaining contact with the commutator.\
How does a repulsion motor generate torque?
- Torque is generated in a repulsion motor when the brush axis is positioned at an angle that is neither aligned nor perpendicular to the magnetic axis of the stator poles. This positioning induces voltage, creating armature current, which interacts with the magnetic field and results in torque.
What is repulsion start induction motor?
- A repulsion-start induction-run motor starts as a repulsion motor but transitions to function as an induction motor at a constant speed. It includes a centrifugal mechanism that short-circuits the commutator once approximately 75% of the full speed is reached, allowing the motor to run as an induction motor.
Why are the brushes short-circuited in a repulsion motor?
- Brushes in a repulsion motor are short-circuited to facilitate the flow of current in the armature and help develop torque by creating the necessary electromagnetic interactions between the rotor and stator.
Is a ceiling fan motor and repulsion motor the same?
- No, a ceiling fan motor is not the same as a repulsion motor. Ceiling fans typically use single-phase induction motors, which operate on different principles compared to the repulsion motor's unique torque generation and commutator mechanism.
What are the main types of repulsion motors?
- Standard Repulsion Motor: Operates solely on the repulsion principle.
- Compensated Repulsion Motor: Includes a compensating winding and additional brushes.
- Repulsion-start Induction-run Motor: Starts as a repulsion motor and transitions to an induction motor.
- Repulsion Induction Motor: Combines features of both repulsion and induction principles with dual rotor windings.
How is speed controlled in a repulsion motor?
- Speed control in a repulsion motor can be achieved by shifting the brushes, which changes the interaction between the rotor and stator magnetic fields.
What are some key characteristics of repulsion motors?
- High starting torque (up to 350%).
- Moderate starting current (3 to 4 times the full-load value).
- Speed varies with load and can become dangerously high at no load.
- Issues with brush sparking may occur.
- Good power factor at high speeds but low power factor at low speeds.
Why is brush positioning critical in a repulsion motor?
- Proper brush positioning ensures optimal torque generation. If the brush axis is incorrectly aligned, no torque is developed, and the motor will not start or run effectively.
What are common issues related to repulsion motors?
- Common issues include brush sparking, speed variations with load, and low power factor at low speeds.